A traditional magnetic storage device relies on a track-following architecture in which the tape drive attempts to follow a previously written track when reading it back by maintaining a very precise alignment between the path traced by the read heads and the written tracks on a tape.
In a track-following architecture, the read channel circuitry employs a phase locked loop (PLL) which locks a synchronization clock signal to the read signal (i.e., the incoming data from the read heads) in order to properly perform data detection. In track-following storage devices, the PLL acquires lock of the clock synchronization signal once at the beginning of the read session, and maintains lock for the entire session.
Recently, non-track-following storage devices have been developed. In these non-tracking storage devices, the previously written track is not followed continuously. Instead the read head may begin on one track and drift over to an adjacent track during the read operation. In this situation, the read signal will degrade during the crossover period, and dock synchronization may be lost. If the frequency of the clock synchronization signal drifts too far during this crossover period, it will prevent reacquisition of lock when approaching the next adjacent track. A similar effect can happen when reading through a long magnetic defect on the tape.
It is difficult to reliably detect when the read heads are deviating from a track based solely on the read head signal amplitude. If the read head is partially over the track that it is departing from, and in addition is partially over an adjacent track being approached, the overall signal amplitude may not be detectably reduced. However, the signal quality would prevent the data from either track from being successfully read.
Accordingly, a need exists for a method for detecting when a read head is moving off track and for reacquiring clock synchronization in a non-tracking storage device when the read head moves off track.